The unmanned Apollo 6 Command Module (right) was launched from Kennedy Space Center on April 4, 1968 and recovered (center) in the Pacific Ocean 10 hours later. The mission was the second test flight for the giant Saturn V (left) launch vehicle and the last unmanned flight of Project Apollo.

The Command Module is on display in the FSC exhibit hall!

Apollo 6 Mission Summary

The Apollo 6 mission provided a second rehearsal for launching the massive Saturn V rocket. Scientists and engineers were testing the “staging” of a giant rocket to be sure each section would work properly. An important mission objective was to check out all systems before sending astronauts into space. The vehicle carried a full payload, including a mock-up lunar module, and was to test the capsule’s heat shield to see if it could withstand re-entry speeds.

Initially, the launch seemed to be fine. But approximately two minutes into the flight, the first stage’s five F-1 engines developed serious thrust fluctuations that caused the rocket to bounce like a pogo stick for 30 seconds. These oscillations were so intense that an airborne chase-plane’s cameras recorded pieces of the adapter stage (housing the lunar module) falling off of the vehicle. Such low-frequency vibrations (known as “pogo effect”) exceeded the engineering / safety design criteria of the Apollo 6 Command Module. Had astronauts been onboard the spacecraft, the mission would have been aborted by jettisoning the capsule away from the failing rocket.

Although the oscillations stopped once the first stage was discarded, the vehicle’s second stage performance was also less than perfect. Two of the stage’s five J-2 engines failed, causing the remaining three engines to burn for a longer period of time than planned. As a result, the second stage ran out of fuel before reaching the desired 100 mile circular orbit.

To compensate, the Saturn’s third stage burned longer and placed the spacecraft into an unplanned 110 by 230 mile elliptical orbit. NASA engineers left Apollo 6 in this “parking orbit” for two revolutions around the Earth to assess the situation and perform various system checks. When flight controllers attempted to fire the third stage again, to simulate the flight to the Moon, the J-2 engine failed to restart as planned.

The issues with the Saturn V’s three stages altered the mission, and it was decided that after separation from the third stage, the Service Module’s engine would burn for seven minutes, pushing the Apollo 6 capsule to an altitude of almost 14,000 miles. At such an altitude, enough re-entry speed could then be acquired to simulate an Apollo spacecraft returning from the Moon. The capsule’s heat shield withstood the fireball created by a 22,000 mile per hour plunge into the Earth’s atmosphere. Apollo 6 splashed down in the Pacific Ocean, completing its 10 hour perilous space odyssey, and was recovered by the crew of the U.S.S. Okinawa.